1. Field of the Invention
The present invention relates to recording apparatus and method, and more particularly to ink jet recording apparatus and method for discharging fine ink droplets toward a recording medium such as a sheet to record characters or images thereon, and further particularly to recording apparatus and method which use as many nozzles as the number corresponding to a recording width of the recording medium.
2. Related Background Art
An ink jet recording apparatus for discharging fine ink droplets to record data has been known. This recording apparatus has many advantages over other recording apparatuses such as high recording speed, ease of colorization, plain paper recording, low noise and high recording quality.
Such an ink jet recording apparatus has a recording head which comprises discharge ports (or outlets) for discharging inks, nozzles connected to the discharge ports and energy generation means arranged at portions of the nozzles to generate energy to discharge the inks in the nozzles. The recording head selectively discharges ink droplets from the discharge ports in accordance with input record information to form characters and images on the recording medium.
However, in such a prior art ink jet recording head, since a plurality of nozzles communicate with a single common liquid chamber, nozzles mutually interfere with each other and a discharge characteristic of the ink droplets is deteriorated.
The mutual interference means an affect of the first discharge to the second discharge from the recording head when
1 ink is discharged from a particular nozzle at the first discharge, and
2 ink is discharged from a nozzle adjacent to the nozzle of 1 at the second discharge. Namely, it means a phenomena in which the quantity of discharge of the ink or the velocity of discharge of the ink at the second discharge changes between the operation 2 immediately after the operation 1 and the operation 2 without the operation 1 or sufficiently long interval after the operation 1.
The deterioration of the discharge characteristic means that a change in the quantity of discharge of the ink or the velocity of discharge is large so that the quality of the characters or images is deteriorated. The change in the quality of discharge of the ink has a great affect to the quality. The larger the number of nozzles which concurrently discharge the ink in the operations 1 and 2 is and the shorter a distance between a rear end of a separation wall of the nozzles and a rear surface of the common liquid chamber in the structure of the recording head is, the more remarkably does the phenomenon occur.
The causes of the mutual interference are explained with reference to FIGS. 3A and 3B which illustrate the structure of the recording head and FIGS. 4A to 4C which illustrate the mutual interference between the nozzles. FIGS. 3A and 3B show an ink jet (bubble jet) recording head. Numeral 300 denotes a resistor which is an energy generator, numeral 301 denotes a nozzle, numeral 302 denotes a discharge port, numeral 303 denotes a common liquid chamber, numeral 304 denotes a filter, numeral 305 denotes a silicon (Si) substrate, numeral 306 denotes an aluminum (A1) substrate, numeral 307 denotes an ink supply tube, numeral 308 denotes a flexible cable and numeral 309 denotes a nozzle separation wall. FIG. 4A shows a timing t=0 to conduct a first discharge of the operation 1 by a first pulse P1 and a timing t=t.sub.s to conduct a second discharge of the operation 2 by a second pulse P2. Each of P1 and P2 has a pulse width of 10 .mu.sec and a voltage applied to the resistor is 30V. FIG. 4B shows a portion of FIG. 3B and illustrates the propagation of an ink pressure at a time immediately before the drive of the heat generators of the nozzles No. 5 to No. 8 by the first pulse P1.
By the heat generated by the heat generators #1 to #4 (or No. 1 to No. 4), bubbles 321 are generated and the ink is discharged in the direction C. At the same time, a small amount of ink flows back to the common liquid chamber 303 as shown by an arrow D. By this phenomenon, a small amount of ink extends in the direction of discharge, although the ink is not fully discharged in the direction E, from the discharge ports 302 of the nozzles #5 et seq. Namely, as shown in FIG. 4C, a meniscus 322 which is an interface between the ink and atmosphere exhibits slightly convex. Thereafter, the nozzles #1 to #4 normally discharge the ink.
However, when the pulse P2 is applied at t=t.sub.s =13 .mu.sec to discharge the ink from the nozzles #5 to #8, the discharge is conducted while the meniscus 322 is convex, and the volume of discharge of the ink from the nozzles #5 to #8 is larger by .DELTA.V than that when the pulse P2 is applied without the pulse P1. Namely, larger ink droplets are discharged.
It has been known that when the quantity of discharge of the ink of the adjacent recording point changes approximately 10%, the deterioration of the record quality can be visually recognized.
It is difficult to precisely measure the increment of the quantity of discharge of the ink from the nozzle #5, it is estimated as follows by approximation calculation. In the condition of FIG. 4B, the extension of the meniscus 322 from the nozzle #5 is 10 .mu.m. The actual nozzle sectional size is 20 .mu.m.times.25 .mu.m but it is approximated by a cylinder having a diameter of 25 .mu.m. Further, since the extension of the meniscus 322 is approximated to a portion of a sphere as shown in FIG. 4C by the microscope observation, the increment by the extension of the meniscus 322 is given by EQU .DELTA.V1=2.98 (pico liters)
Under the non-discharge condition of the ink, the meniscus 322 is set to be slightly convex and recessed by 2 .mu.m. A distance (or difference) to the tip end of the nozzle is given by EQU .DELTA.V2=0.16 (pico liters)
Thus, the increment of the quantity of discharge of the ink is given by EQU .DELTA.V=.DELTA.V1+.DELTA.V2=3.14 (pico liters)
Since the normal quantity of discharge of the ink is approximately V=28 (pico liters), the change of the ink droplets is EQU .DELTA.V/V=11.2 (%)
and the recording quality is deteriorated.
In the above description, it is assumed that the position of the meniscus 322 of the nozzle #5 changes with time and the pulse P2 is applied at the most extended timing. However, it has been proved that the change in the quantity of the ink droplets is larger when the pulse P2 is applied at a slightly earlier timing. This is considered because there is a time lag from the application of the voltage to the heat generator to the generation of the bubbles on the heat generator and the force which the ink receives in the direction of discharge is larger when the meniscus 322 moves toward the extension than when the meniscus 322 is at the most extended state.
When the pulse P2 is applied around t=40 .mu.sec, the change in the quantity of the ink droplets is negative as opposed to the above case. This is considered because the meniscus 322 which has once been convex recovers by a surface tension of the ink and becomes concave rather than the normal state by a kinetic energy, and the arrows D and E in FIG. 4B are of opposite direction at the timing of the extinguishment of the bubbles generated at the nozzles #1 to #4.
One solution to those problems is disclosed in U.S. Pat. No. 4,578,687. In this method, an atmosphere opening for the ink is provided at a portion of the recording head to divert the change in the pressure in the common liquid chamber to the atmosphere when a particular nozzle discharges the ink to prevent the interference to other nozzles. This method, however, has disadvantages of introduction of fine dusts from the atmosphere opening, the inducement of non-discharge of the ink due to the change of material property by the evaporation of the ink, and the inability of the discharge due to the adhesion of the ink. In order to avoid those problems, the apparatus should be very complex.
Means for retaining bubbles for the interference of the pressure of a portion of the common liquid chamber is disclosed in Japanese Laid-Open patent application Ser. No. 1-285356 but it is difficult to always retain a constant volume of bubbles.
As an approach to minimize the occurrence of the problem due to the mutual interference, it has been proposed to make a length d from a rear of the energy generator 300 shown in FIG. 3B to a rear of the common liquid chamber 303 sufficiently long. In an experiment, the mutual interference was minimized by setting the length d longer than 6.0 mm. However, this method is against a desire to reduce the size of the recording head to make the recording apparatus compact. Further, since the Si substrate is an expensive component, the increase of the size of the recording head leads to the increase of the cost of the apparatus.
As a method for preventing the mutual interference by the vibration of the meniscus of the non-discharge nozzle, a method of simultaneously driving all nozzles has been proposed. However, in this method, the larger the number of nozzles is, the larger is the capacity of the driving power for the resistor which is the energy generator. As a result, the size of the apparatus increases and the cost of the apparatus increases.
The mutual interference may also be prevented by providing a time interval after the discharge from one nozzle to the discharge timing of the adjacent block which is long enough for the meniscus to return to the normal state rather than convex or concave. However, this method is in contrast to the requirement of high recording speed and a step appears prominently in the recorded characters and images due to the lag in the discharge timing and the recording quality is deteriorated.